Sm318 to Sm420 swap in 59GMC
I would assume its a pretty easy swap im getting sick of the first gear in the 318. I know the tranny hump will need to be replaced with the sm420 one but anything else? Driveshaft length the same? Mounts?
|
Re: Sm318 to Sm420 swap in 59GMC
Depending on the three speed, you may find the bottom bolt holes are a problem. Light duty three speeds were bolted to the bellhousing from the rear. Heavy duty three speeds and 4 speeds are different. Upper trans to bellhousing bolts are installed from the rear while the lower bolts are installed from the front, inside the bellhousing. SM318 Should be a light duty transmssion.
Driveshaft may not be the same. Does your transmission currently have a slip yoke or bolt on rear yoke? How about the one you want to install? If you can find a heavy duty three speed it has different gear ratios in 1 and 2. 1st 3.17 2nd 1.75 3rd 1.00 R 3.76 |
Re: Sm318 to Sm420 swap in 59GMC
2 Attachment(s)
You will have to change the driveshaft as the transmisisons aren't even close to being the same as far as length or the way the driveshaft hooks up
Info on the 420 here https://www.novak-adapt.com/knowledg.../manual/sm420/ As far as gears, no one I know ever starts out in low gear (granny) with an SM 420 unless they are pulling a grade or pulling a heavy load. 90 % of the time you start out in second gear. Second gear is 3.6 though and is better than either first gear offered in the 318 as there are two first gear ratios on the 318, 1st gear ratio Suffix A equals 3.03:1, Suffix B equals 2.42:1. 318 on left, 420 on right. I'm thinking with the truck bellhousing you use a bolt and nut with the 318 on the two lower bolts and the two lower bolts on the 420 thread into the two bottom bolts on the trans. |
Re: Sm318 to Sm420 swap in 59GMC
Quote:
|
Re: Sm318 to Sm420 swap in 59GMC
Quote:
If you have the SM420 then by all means use it. But it's noisy in 1, 2, and 3 (direct drive is very quiet). Very noisy. I ended up adapting a shifter from a GMT400 so I could use the rubber isolated base to keep gear noise away. Mr48 is right about Creeper gear being nearly useless. If you have to buy the transmission I would look for an SM465 In some ways it's the "Hey, we could have done this better" version of the SM420. You might luck out and find a fully synchronized 3 speed. In trucks they started around 65-66 depending on the transmission model. Good luck, and don't forget to update this thread if you swap the trans! |
Re: Sm318 to Sm420 swap in 59GMC
Quote:
|
Re: Sm318 to Sm420 swap in 59GMC
1 Attachment(s)
I've got a full syncro Saginaw 3 speed out of a later Chevy truck in my 48 and that has been one smooth working transmission. My buddy gave/traded it to me years back when the Muncie 3 speed I had in it needed serious work.
It is similar to this one out of a 68 Chevy and they came in 66 to early 70's Chevys. |
Re: Sm318 to Sm420 swap in 59GMC
Quote:
|
Re: Sm318 to Sm420 swap in 59GMC
68/ though early 70's cars and pickups is what they came in.
The best part is that unlike the 318 you can shift into low smoothly while still rolling up to a light. They shouldn't be all that hard to find in the Fargo area as there should have been plenty of them sold in pickups in that area around that time. The "I want a new truck but don't want to pay for a bunch of extras" trucks. I've got some extended family in the Fargo area either Davison or Dunn. |
Re: Sm318 to Sm420 swap in 59GMC
Quote:
|
Re: Sm318 to Sm420 swap in 59GMC
Sorry, I gave you all the info I have and you will just have to do the same searching that I would have to do to give you an answer.
Maybe I should put that in my signature at the bottom of the page, = I don't know everything, but I do know how to search effectively. |
Re: Sm318 to Sm420 swap in 59GMC
Quote:
|
Re: Sm318 to Sm420 swap in 59GMC
|
Re: Sm318 to Sm420 swap in 59GMC
Quote:
|
Re: Sm318 to Sm420 swap in 59GMC
Quote:
1968 three speed site:67-72chevytrucks.com BTW, it really looks like the Saginaw 3 speed transmission is the one to get. Searching FB Marketplace, Craigslist, and Ebay all get results for that. Good luck. |
Re: Sm318 to Sm420 swap in 59GMC
You don't want the 55/60 Saginaw 3 speed it is the 66 and up transmission that is the desirable one.
This chart shows the Saginaw 3 speed 343 and the Muncie 351 transmissions both with 7 bolt side covers and fully syncro. https://documents.holley.com/techlibrary_manual.pdf I can find the Sanginaw 343 listed for Camaros and listed for trucks and cars but no actual definitive list of donor rigs and have been searching for about a half hour. All I can really say about them is that they shift smooth and they are heavy. Heavy when you are laying under the truck on your back and have to stab one by hand without help and I did that more than once with mine but probably couldn't manage it now. |
Re: Sm318 to Sm420 swap in 59GMC
So I pulled the engine and trans out yesterday, the tranny bolts to the bell housing on top from the outside the bell housing and the bottom from inside. Also not sure if this 235 bell housing will even fit my 270 now either so might be getting a new one of those.
|
Re: Sm318 to Sm420 swap in 59GMC
The truck bellhousings are like this. Two bolts from back on top, two on bottom from front. Car bellhousing is all four from back. Mixing and matching can mean using nut and bolt combination or, maybe (I've never tried this) helicoil. Also need to watch the diameter of the large center hole which locates the transmission using the input shaft bearing retainer. Most transmissions and bellhousings from this era match up but once in a while they don't.
|
All times are GMT -4. The time now is 07:48 PM. |
Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2024, vBulletin Solutions Inc.
Copyright 1997-2022 67-72chevytrucks.com